1. Field of the Invention
This invention relates to an optical pickup device for recording information signals on an information signal recording medium, such as a magneto-optical disc. More particularly, it relates to an optical pickup device in which a light beam radiated from a light source is radiated via an objective lens on the information signal recording medium and the return light beam reflected from the recording medium is detected by optical detection means for reading the information signals recorded on the recording medium.
2. Background of the Invention
There has hitherto been proposed an information recording medium for recording information signals by optical means. Typical of the information recording media is a magneto-optical disc. The magneto-optical disc is made up of a disc substrate and a signal recording layer deposited on the disc substrate. The signal recording layer is formed of a material the direction of magnetization of which may be inverted on heating to a temperature not lower than the Curie temperature by radiation of a laser beam and by application of an external magnetic field. For recording and reading information signals on or from the information recording medium, an optical pickup device as explained hereinbelow is employed.
The optical pickup device includes a laser diode device 101 as a light source and an objective lens 102 for converging a light beam L radiated by the laser diode device 101 on the signal recording layer, as shown in FIG. 1. Besides, the optical pickup device includes a photodetector 103 for detecting a return light beam Lr of the light beam L once converged on the signal recording layer.
The laser diode device 101 has a can-shaped package and is driven into laser oscillation by a driving current supplied via plural terminals. The laser diode device 101 has a semiconductor chip arranged in the package and radiates a light beam from the terminal end of an active layer of the semiconductor chip as a result of the laser oscillation. The laser diode device 101 is substantially a point light source. The light beam radiated by the laser diode device 101 is collimated by a collimator lens 104. The collimated light is incident on and transmitted through a beam splitter 105 so as to be incident on the objective lens 102. The objective lens 102 converges the light beam L on a signal recording surface 106a which forms a boundary between the signal recording layer and the disc substrate of the magneto-optical disc 106.
On the signal recording surface 106a, the information signals are substantially concentric in shape and written along a spiral recording track on the recording surface.
The light beam converged on the signal recording surface 106a is reflected back thereby so as to be re-incident on the objective lens 102 as a return light beam Lr. The return light beam Lr, re-incident on the objective lens 102, is substantially collimated and re-incident in this state on the beam splitter 105. The major portion of the return light beam re-incident on the beam splitter 105 is reflected back from a multi-layer dielectric film 107 of the beam splitter 105 so as to be incident on a three-beam Wollaston prism 108. The multi-layer dielectric film 107 of the beam splitter 105 is so designed as to transmit approximately 80% of a P-polarized component and to reflect approximately 95% of an S-polarized component of the return light beam L r.
The return light beam Lr incident on the three-beam Wollaston prism 108 is transmitted therethrough and converged by a converging lens 109 on the light receiving surface of the photodetector 103.
The photodetector 103 has plural light receiving surfaces and is arranged within the package. The photodetector 103 is adapted for converting the intensity of the return light beam received by the light receiving surfaces into corresponding electrical signals and for outputting the electrical signals at plural terminals associated with the light receiving surfaces. That is, the electrical signals outputted from the photodetector 103 represent changes in the light volume, in the state of polarization or in the amount of astigmatism of the return light beam.
If the optical pickup device causes information signals to be recorded on the magneto-optical disc 106, the light beam L outgoing from the laser diode device 101 is condensed and radiated on the signal recording layer for locally heating the signal recording layer. At this time, an external magnetic field is applied by a magnetic head device 110 on the signal recording layer. In addition, the state of light convergence of the light beam on the signal recording layer and the position of light beam radiation on the signal recording layer are adjusted at this time based on the various information obtained on the basis of output electrical signals of the photodetector 103.
The information signals recorded on the magneto-optical disc 106 may be read by the optical pickup device based on the information derived from the photodetector 103 while the outgoing light beam from the laser diode device 101 is converged and radiated on the signal recording layer.
The multi-layer dielectric film 107 of the beam splitter 105 of the conventional optical pickup device described above has a polarization film characteristic of causing substantially 100% of the S-polarized component of the return light beam from the magneto-optical disc 106 to be incident on the photodetector 103. However, such polarization film characteristic presents a problem that it is significantly changed depending on the angle of incidence of the light beam L on the beam splitter 105.
That is, if the beam splitter 105 is arranged in a converging light beam or in a divergent light beam, double refraction is likely to be incurred in the multi-layer dielectric film 107 of the beam splitter 105. If double refraction is incurred in the multi-layer dielectric film 107, the carrier to noise (C/N) ratio is lowered so that information signals cannot be read satisfactorily from the magneto-optical disc 106. Such double refraction is ascribable to the incident angle dependency of transmittance to the incident light beam of the multi-layer dielectric film 107 of the beam splitter 105 and is incurred in particular due to the phase deviation between the P-polarized component and the S-polarized component produced by the multi-layer dielectric film 107 insofar as the convergent and divergent light beams are concerned.
Consequently, it has been customary with the conventional optical pickup device to arrange the beam splitter 105 in a portion on the light path between the laser diode device 101 and the objective lens 102 in which the light beam L remains a collimated light beam, and to arrange the three-beam Wollaston prism 108 so that the collimated light reflected by the multi-layer dielectric film 107 of the beam splitter 105 is incident thereon. That is, with the above-described conventional optical pickup, it becomes necessary to provide the collimator lens 104 upstream of the beam splitter 105 for collimating the divergent light beam, and the converging lens log for converging the collimated light transmitted through the three-beam Wollaston prism 108 to the photodetector 103. The result is an increased number of the component parts and an increased length of the optical path, thus leading to increased production costs and hindrance to the reduction in size and simplification of the optical pickup device.
For reducing fluctuations in polarization film characteristic due to the variable incident angle of the light beam L on the beam splitter 105, there is proposed a method for adjusting the polarization film characteristic of the multi-layer dielectric film of the beam splitter 105 so that the reflectance of the P-polarized component Rp is equated to the reflectance of the S-polarized component Rs (Rp=Rs). However, this raises a further problem that the effect of the beam splitter 105 on increasing the angle of rotation of the plane of polarization, known as an enhancement effect, cannot be expected, such that the C/N characteristic of the photomagnetic signals is lowered.
The above-mentioned enhancement effect is the effect that the angle of rotation .phi. of the plane of polarization caused by the magneto-optical disc 106 becomes larger by a factor of .alpha. after reflection. The factor o may be given by an equation 1 ##EQU1## The larger the amounts of changes in the angle of rotation .phi. of the plane of polarization of the incident light and an angle of rotation .alpha..phi. of the plane of polarization of the reflected light, the higher becomes the detection characteristic of the S-polarized component and the P-polarized component on the photodetector 103 to improve the C/N characteristic of the photomagnetic signals.
However, since the relation Rp=Rs needs to be met in the above-described proposed optical pickup device, the value of a is diminished, as a result of which the C/N characteristic of the photomagnetic signals are deteriorated.